[BUG] There are quite some bug reports of btrfs falling into a ENOSPC trap, where btrfs can't even start a transaction to add new devices. [CAUSE] Most of the reports are utilize multi-device profiles, like RAID1/RAID10/RAID5/RAID6, and the involved disks have very unbalanced sizes. It turns out that, the overcommit calculation in btrfs_can_overcommit() is just a factor based calculation, which can't check if devices can really fulfill the requirement for the desired profile. This makes btrfs_can_overcommit() to be always over-confident about usable space, and when we can't allocate any new metadata chunk but still allow new metadata operations, we fall into the ENOSPC trap and have no way to exit it. [WORKAROUND] The root fix needs a device layout aware, chunk allocator like available space calculation. There used to be such patchset submitted to the mail list, but the extra failure mode is tricky to handle for chunk allocation, thus that patchset needs more time to mature. Meanwhile to prevent such problems reaching more users, workaround the problem by: - Half the over-commit available space reported So that we won't always be that over-confident. But this won't really help if we have extremely unbalanced disk size. - Don't over-commit if the space info is already full This may already be too late, but still better than doing nothing and believe the over-commit values. CC: stable@xxxxxxxxxxxxxxx # 4.4+ Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> --- fs/btrfs/space-info.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c index 475968ccbd1d..e8133ec7e34a 100644 --- a/fs/btrfs/space-info.c +++ b/fs/btrfs/space-info.c @@ -339,6 +339,18 @@ static u64 calc_available_free_space(struct btrfs_fs_info *fs_info, avail >>= 3; else avail >>= 1; + /* + * Since current over-commit calculation is doomed already for + * RAID0/RADI1/RAID10/RAID5/6, we half the availabe space to reduce + * over-commit amount. + * + * This is just a workaround before the device layout aware + * available space calculation arrives. + */ + if ((BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1_MASK | + BTRFS_BLOCK_GROUP_RAID10 | BTRFS_BLOCK_GROUP_RAID56_MASK) & + profile) + avail >>= 1; return avail; } @@ -353,6 +365,14 @@ int btrfs_can_overcommit(struct btrfs_fs_info *fs_info, if (space_info->flags & BTRFS_BLOCK_GROUP_DATA) return 0; + /* + * If we can't allocate new space already, no overcommit is allowed. + * + * This check may be already late, but still better than nothing. + */ + if (space_info->full) + return 0; + used = btrfs_space_info_used(space_info, true); avail = calc_available_free_space(fs_info, space_info, flush); -- 2.28.0
